netfilter: nfnetlink_log: fix possible compilation issue due to missing include

In (0c36b48 netfilter: nfnetlink_log: fix mac address for 6in4 tunnels)
the include file that defines ARPD_SIT was missing. This passed unnoticed
during my tests (I did not hit this problem here).

net/netfilter/nfnetlink_log.c: In function '__build_packet_message':
net/netfilter/nfnetlink_log.c:494:25: error: 'ARPHRD_SIT' undeclared (first use in this function)
net/netfilter/nfnetlink_log.c:494:25: note: each undeclared identifier is reported only once for
+each function it appears in

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nfnetlink_log.c

@@ -13,6 +13,7 @@
  */
 #include <linux/module.h>
 #include <linux/skbuff.h>
+#include <linux/if_arp.h>
 #include <linux/init.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>